Colour sensing for the LED market is an emerging technology as companies are considering incorporating solutions into their portfolios. Conversely, the LED market is a fairly mature market with various manufacturers that include makers of LED modules, LED engines, luminaires, smartphones, flat panel TVs, laptops, etc. These manufacturers follow the performance standards established by a number of organizations including ANSI/ANSLG, CIE, IES, and NEMA for LED component manufacturers, luminaire manufacturers, etc. Many customers express a desire of having a colour sensor that can support high accuracy, e.g. a Δu′v′ of 0.001 to 0.002 which implies a very tight device-to-device tolerance. Given the complexity of transforming, for example, RGB counts per μw cm2 to CIE XYZ tristimulus colour space, decreasing the variance will not likely by itself address the desired degree of accuracy.
In general, all possible variances of a device must be minimized in order to increase detection accuracy. The device-to-device accuracy is expected to be in terms of colour temperature, Duv, or Δu′v′. Ultimately, the market is demanding that the colour sensor correspond to the CIE standard colour-matching function with a very high degree of repeatability.
Typically, standard process manufacturing which includes common process and non-proprietary filter material is employed in order to keep the device cost low. Unfortunately, the colour filter spectral curves do not match the standard colour-matching function of the CIE standard, for instance. In addition, the colour filters are affected by their absorption coefficient, filter thickness, and concentration. The infrared (IR) deposit typically also has a “rippling effect” with peaks and troughs that affect the RGB colour response as well. Furthermore, there are other silicon related effects that impact the device-to-device performance, for example temperature coefficient, that must be addressed to minimize the Δu′v′ resolution across the entire operating temperature of the device.
Currently, efforts to minimize device-to-device tolerance are typically addressed either through wafer or post-package trimming or else by testing and binning devices. Temperature compensation techniques are also employed to improve device accuracy. Colour space transformations are virtually always performed off-device. As a result, device-to-device tolerance for a widely accepted (or custom) colour space transformation can vary from part to part if the testing environment is not tightly controlled by a third-party. There is no current solution that is able to provide very accurate device-to-device tolerances natively for third-party colour transformations.